Exhalation valves have been used on filtering face masks for many years and have been disclosed in, for example, U.S. Pat. Nos. 4,981,134, 4,974,586, 4,958,633, 4,934,362, 4,838,262, 4,630,604, 4,414,973, and 2,999,498. U.S. Pat. No. 4,934,362 (the '362 patent), in particular, discloses a unidirectional exhalation valve that has a flexible flap secured to a valve seat, where the valve seat has a rounded seal ridge with a parabolic profile. The elastomeric flap is secured to the valve seat at the apex of the parabolic curve, and rests on the rounded seal ridge when the valve is in a closed position. When a wearer of a face mask exhales, the exhaled air lifts the free end of the flexible flap off the seal ridge, thereby allowing the exhaled air to be displaced from the interior of the face mask. The '362 patent discloses that an exhalation valve of this construction provides a significantly lower pressure drop for a filtering face mask.